1. Field of the Invention
The present invention relates in general to an apparatus for testing a lens module, more specifically, to more precise determination of the quality parameter of a lens module used in a system of reflective type object.
2. Description of the Related Art
A scanner is widely used as an image input device for a personal computer. Referring to FIG. 1, a common form of a scanner comprises a glass plate 12 on which an object 11 (generally is a document) is disposed, a light source 13 emits the light to pass through the glass plate 12 to generate an object light reflected from the object 11. The object light is further reflected by a mirror 14 (generally comprises several mirrors in sequence, herein represented by one mirror briefly). Then an image is formed on a charge coupled device(CCD) 16 by passing the reflected object light through a lens module 15, and the CCD 16 produces an electrical signal corresponded to the object 11. The light source 13, the mirror 14, the lens module 15 and the CCD 16 are disposed in the same module, so they always move simultaneously with same speed toward the same direction to keep object length and image length fixed while scanning.
In the process of mass production of lens modules, different aberrations are introduced into different lens modules, and the quality parameters of lens modules may be affected. Referring to FIG. 2, that is a conventional testing system for measuring quality parameters of lens modules to determine whether each lens module is acceptable or not for installation into a scanner. The lens module 15 is mounted on a holding device 24. An emulated object light is formed by the light emitting from a light source 21 passing through a grating 22, and then the testing image of the grating 22 is projected onto CCD 25 by the lens module 15. Thus, the testing process is as follows:
(1) putting the grating 22 and the CCD 25 at the positions around the object location and image location of the lens module 15 respectively; PA1 (2) rotating the lens module 15 around its optical axis to get the MAX modified MTF and fixing the lens module 15 on that angle; PA1 (3) moving the CCD 25 along the optical axis back and forth to measure the magnification of the lens module 15; PA1 (4) comparing all the quality parameters with acceptable specification to determine whether the lens module 15 is acceptable or not.
In Taiwanese patent application serial No. 85109212, the detail of the operation of the conventional testing system is described. MTF and magnification is calculated following the below equations: EQU MTF=(the max. value of gray level-the min. value of gray level)/256*100% EQU Magnification=length of image/length of object=(CCD dot number corresponding to image*length of CCD dot)/length of object
However, the lens module testing system described above contains the disadvantages described below:
First, during the real scanning operation, the distance between object and CCD is fixed. However, in the testing system, the CCD can be moved for fitting to the different focal length of different lens modules. That will cause some test errors.
Second, during the real scanning operation, the object light is reflected by a document; however, the emulated object light used in testing system is formed by the light passing through the grating. Since these two lights are generated in different ways, that certainly causes test errors.